Typical fuel rods for nuclear reactors comprise a cladding tube, and fuel pellets are inserted into the cladding tube together with a spring for holding the fuel pellets in position. Each fuel rod is pressurized with a gas, typically helium, and the fuel rod is subsequently plugged. The fuel rod is plugged by means of a plug that is pressed into the cladding tube. The plug is sealed, normally by means of welding, to the cladding tube, in order to ensure that the pressure inside the fuel rod is maintained. However, such welding may be performed at a lower surrounding pressure than the pressure inside the fuel rod. A plugged fuel rod may leak gas before the plug has been sealed, and thereby loose some pressure. A fuel rod with too low pressure may not function properly and may also cause damage when operating in the nuclear process. It is therefore important to ensure that the pressure inside the fuel rod is maintained at a proper level after the sealing of the fuel rod.
U.S. Pat. No. 5,319,178 (“Sando”) illustrates a welding apparatus for welding plugs to fuel rods, which apparatus holds the fuel rod and applies a pressure to the fuel rod during welding. FIG. 7 in document “Sando” illustrates a fuel rod P being held by a holding mechanism F3, and a plug C is welded to the top of the fuel rod P inside welding chamber 18. A pressing mechanism F1 is used to apply pressure to the connection between the fuel rod P and the plug C during the welding. FIG. 6, in document “Sando”, illustrates in more detail a welding torch 16 inside the welding chamber 18, which welding torch 16 is used when welding the plug C to the fuel rod P. The fuel rod is rotatably arranged to the welding chamber 18 and is rotated during the welding.
U.S. Pat. Nos. 4,925,619 and 4,181,008 show two systems for monitoring the internal gas pressure of a fuel rod.
U.S. Pat. No. 4,925,619 discloses an apparatus for sensing the internal pressure of a fuel rod. The apparatus comprises a strain gauge that senses the increase in diameter of the fuel rod that is caused by the pressurization of the fuel rod.
U.S. Pat. No. 4,181,008 describes a method for assuring the pressure of a fuel rod during manufacturing of the fuel rod. This method utilizes a system of external chambers for determining whether gas leaks from the fuel rod after welding, wherein pressure sensors measure the pressure in the external chambers.